Ice skates provide a narrow band of contact between each foot and the ice. The area of contact with the ice is ideally small such that the ice is melted by the pressure and this miniscule water layer adds to the slip of the skate contact area over an ice surface. The small area in contact with the melt by itself would be counterproductive since unlimited slip would also provide no control over direction or speed. A second element is needed, at least one element which can penetrate the water layer and act upon the solid ice to provide for directional changes, acceleration force or braking force. In normal ice skates the bottom, at one time a flat narrow band of steel, is now a concave strip of steel in contact with the ice with the long edges along a blade and a ground away center between these edges of the ice skate blade providing for a water cushion between two sharp edges that easily contact solid ice.
In practice the skate blade requirements are highly conflicted. The projecting edges must be very hard because, contrary to intuition, ice is very abrasive. If the edges are not hard, wear rapidly decreases the “bite” of the edges into the solid ice. There is also a contrary need for flexibility of the blade. Hardness is generally associated with stiff non-flexible materials while very easily flexed materials are usually soft and deformable. Forces transverse to the long axis of the blade during acceleration or braking where the edges bite into the ice at an angle normal to the direction of travel place extreme shear forces upon the blade. Since hard materials tend to be brittle the shear forces easily break hard blades. A delicate balance is needed to obtain a hard edged blade which will not break in use.
One way to measure the hardness of a steel blade is by using the well known Rockwell C scale of hardness. This used an indenter to measure and provide a numerical value representing the resistance of the steel to the penetration of a steel indenter which is easily related to hardness. A very hard alloy steel can be 65 RockwellC while untreated hot rolled steel is at 20 Rockwell C. The 65 Rockwell materials, despite the best alloying effects are just plain brittle. A thin strip of this hard a steel will be shattered by a sharp impact. A thin strip of hot rolled steel will merely deform if impacted. Clearly an ideal skate blade would have outside edges along the length of the blade that were close to 65 Rockwell C scale with a center of 20 Rockwell C. There could be several ways to make a blade with a softer center and a hard edge. As noted below, all prior blades that attempted this had serious drawbacks to the hard edge, soft center blade concept. While Rockwell C scale is used throughout in this discussion, the actual measurements of the hardness of the nitrided portions of the skate blade were performed by cutting slices of the blade normal to the flat surface and close to the ice contact surface and measuring these cut and polished surfaces with a microindenter. The microindenter provides Vickers scale results which were converted into the more frequently used Rockwell C scale numbers using an ASTM issued conversion chart.
A first logical method of making an ideal blade is used in Swedish knife manufacture where a hard layer is coupled or bonded to a soft layer. Problem not solved since the hard layer is very brittle and a hard layer backed by a softer layer acts like a very thin glass microscope slide placed on a warm stick of butter. The least pressure normal to the slide surface will shatter it since there is no reinforcement for the brittle glass material. The same thing happens when soft steel and hard steel is combined (although it does work better if the hard material is in the center like the knives). This type blade would have broken edges which would grab the solid ice and ruin control. Additional problems are differential expansion which can warp the blade and the problems of bonding the layers in a long strip without distorting the strip.
If composites do not work, then selective hardening should provide an ideal result. Steels can be surface hardened with heat application. Unfortunately, thin strip heats too fast to provide a softer center with a hard surface. The steels tend to thru harden. The blade is then uniformly brittle or uniformly soft.
Other treatments are also well known. Ancient swords were hot hammered with carbon or boron and folded many times and again beaten flat to form a multi layered structure where microscopic layers of soft and hard provided strength, hardness and flex. This worked on a microscopic scale but a macro (⅛th inch) thickness with just a hard outside and soft center was not obtainable with this method.
Nitriding involves the addition of nitrogen diffused in steel which then precipitates into very hard particles within the steel. Aside from the warping that is caused when the steel has granules added within the structure, the nitrogen diffusion takes place at temperatures that soften steel and the layers are under 8 microns thick so we rapidly get back to brittle thin layers over soft metal—the butter and glass problem revisited.
At the present time, skate blades are through hardened to a Rockwell 40 to Rockwell 55 hardness. This is a compromise where the blade never breaks (at R-C 40) or only once in a while breaks (at R-C 55). The blades need frequent sharpening—to the extent that in Hockey games or competition as many as twice a game/competition. Performance drops as the blade edges start to dull. Sharp edges often are the difference between a win and a loss in the high levels of competition common today.
There are no really useful blades that reliably stay sharp and do not break. The ideal of a hard outer and a flexible soft inner layer may not be possible but new technology allows a reasonable inner layer hardness (Rockwell C 48-50) with gradually increasing hardness within a nitrided layer that ends in hardness well in excess of Rockwell C 65 at the outer (ice contact) edge.